1. Field of the Invention
The present invention is generally directed to electroluminescent (EL) devices, or members, and more specifically, to compounds and organic EL devices using the same, wherein the devices have enhanced thermal and operational stability, and improved durability. The devices of the present invention contain charge, either hole or electron, transport components or compounds comprised of indenofluorene compounds, such as those of the formulas illustrated herein.
2. Discussion of the Related Art
Typical art organic EL devices contain a laminate comprised of an organic luminescent material and electrodes of opposite polarity. As exemplified by U.S. Pat. No. 3,530,325, these devices include a single crystal material, such as single crystal anthracence, as the luminescent substance reference. However, these devices require excitation voltages on the order of about 100 volts or greater. One way to improve device performance has been to incorporate additional layers such as charge injecting and charge transport layers.
Illustrative examples of EL devices are disclosed in publications by Tang et al. in J. Appl. Phys., vol. 65, pp. 3610 to 3616 (1989) and Saito et al. in Mol. Cryst. Liq. Cryst., vol. 253, pp. 125 to 132 (1994), the disclosures of which are fully incorporated herein by reference.
A dual layer organic EL device is typically comprised of one hole transport layer adjacent to the anode supporting hole injection and transport, and an electron transport layer adjacent to the cathode supporting electron injection and transport. In this type of structure, the recombination of charge carriers and subsequent emission of light occurs in one of these layers near their interface. Optionally, a fluorescent material which is capable of emitting light in response to electron-hole recombination can be added to one of said layers.
In another typical configuration, an EL device can be comprised of three separate layers, a hole transport layer, an emission layer, and an electron transport layer, which are laminated in sequence, and are sandwiched as a whole between an anode and a cathode.
Specifically, U.S. Pat. No. 4,356,429 discloses an EL device formed of an organic luminescent medium consisting of a hole transporting layer and an electron transporting layer, wherein the hole transporting layer is comprised of a porphyrinic compound. Further, U.S. Pat. No. 4,539,507 discloses using a substituted aromatic tertiary amine layer for the hole transporting porphorinic layer. Illustrative examples of the aromatic tertiary amine compounds disclosed in the '507 patent are triphenylamines, such as N,N,N-triphenylamine and N,N,N-tri-p-tolylamine, those containing at least two aromatic tertiary amine moieties such as 1,1-bis(4-di-tolylaminophenyl)cyclohexane, and tetraaryidiamines such as N,N′-bis(3-methylphenyl)-N,N′-diphenyl-4,4′-diaminobiphenyl, N,N,N′N′-tetra-p-tolyl-4,4′-diaminobiphenyl. Also, of interest with respect to EL devices are U.S. Pat. Nos. 5,487,953 and 5,554,450.
While hole transport materials comprised of the above-mentioned aromatic tertiary amines are generally known to facilitate hole injection and hole transport processes, the thermal and morphological instabilities of these materials as the hole transport layers have led to relatively poor EL performance and short operational life. There is also a need to develop hole transport materials which are readily accessible synthetically, and which can be prepared in high yields and with excellent electronic purity. Another need resides in the provision of new hole transport materials which are capable of forming thermally and morphologically stable thin films by for example, vacuum deposition techniques. A still further need is the preparation of new hole transport materials suitable for organic EL device applications, and which materials possess excellent hole transport characteristics, enabling the EL devices to operate at low voltages of, for example, below 20 volts. These and other needs can be achievable with the EL devices of the present invention in embodiments thereof.
In U.S. Pat. No. 4,539,507 there is disclosed an EL device formed of a conductive glass transparent anode, a hole transporting layer of 1,1-bis(4-p-tolylaminophenyl)cyclohexane, an electron transporting layer of 4,4′-bis(5,7-di-tert-pentyl-2-benzoxzolyl)stilben, and an indium cathode.
There has been an increased interest in developing energy-efficient flat-panel displays based on organic EL devices primarily because of their potential as an emissive display technology which offers unrestricted viewing angles and high luminescence output at low operating voltages. However, while recent progress in organic EL research has elevated the potential of organic EL devices for widespread applications, the performance levels of a number of current available devices, especially with respect to blue emission, may still be below expectations.
Further, for visual display applications, organic luminescent materials should provide a satisfactory color in the visible spectrum, normally with emission maxima at about 460, 550 and 630 nanometers for blue, green and red. These organic EL devices may comprise a light-emitting layer which is comprised of a host material doped with a guest fluorescent material that is responsible for color emission. For efficient down-shifting of EL emission wavelength in the host-guest emitting layer, it may be desirable that the host material should fluorescence in the blue or shorter wavelength region.
In many conventional organic EL devices, the luminescent zone or layer is formed of a green-emitting luminophor of tris(8-hydroxyquinolinate)aluminum with certain fluorescent materials. U.S. Pat. No. 5,409,783 discloses a red-emitting organic EL device by doping the tris(8-hydroxyquinolinate)aluminum layer with a red fluorescent dye. However, up-shifting of the tris(8-hydroxyquinolinate)aluminum emission to blue region is believed to be highly inefficient.
Although there have been several disclosures describing blue-emitting organic EL devices, for example in U.S. Pat. Nos. 5,151,629 and 5,516,577, the disclosures of which are fully incorporated herein by reference, their performance characteristics still possess many disadvantages such as poor emission hue, high operation voltages, low luminance, and poor operation stability.
Thus, there continues to be a need for improved luminescent compositions for organic EL devices, which may be vacuum evaporable and form thin films with excellent thermal stability. There is also a need for luminescent compositions which are capable of providing uniform and satisfactory emission in the blue region of the light spectrum. In particular, there is a need for efficient blue luminescent materials for organic EL devices, which may optionally be doped with a fluorescent dye. Further, there is also a need for luminescent compositions which can enhance charge transporting characteristics, thus lowering device driving voltages.
The rectification of all these performance deficiencies represents one formidable challenge in EL device research and development. Accordingly, one of the features of the present invention in embodiments thereof is to provide an organic compound, and EL devices using such compound, which provide extended device life span and excellent EL efficiency.